As refiners increase the proportion of heavier crude oil of lower quality in the feed to be treated, it becomes ever more necessary to have particular processes available which are specially adapted to treatment of these residual heavy fractions from oil, shale oil, or similar materials containing asphaltenes and with a high Conradson carbon.
Thus European patent EP-B-0 435 242 describes a process for the treatment of a feed of this type comprising a hydrotreatment step using a single catalyst under conditions which reduce the amount of sulphur and metallic impurities, bringing all the effluent with a reduced sulphur content from the hydrotreatment step into contact with a solvent under asphaltene extraction conditions to recover an extract which is relatively depleted in asphaltene and metallic impurities and sending that extract to a catalytic cracking unit to produce low molecular weight hydrocarbon products. In a preferred implementation in that patent, the product from the first step undergoes visbreaking and the product from the visbreaking step is sent to the asphaltene solvent extraction step. In Example 1 of that patent, the treated feed is an atmospheric residue. According to the teaching of that patent, it appears to be difficult to produce a feed with the characteristics which are necessary to enable treatment in a conventional catalytic cracking reactor with a view to producing a fuel from vacuum residues with a very high metal content (more than 50 ppm, usually more than 100 ppm and normally more than 200 ppm) and with a high Conradson carbon. The current limit on metal content in industrial feeds is about 20 to 25 ppm of metal, and the limit for the Conradson carbon is about 3% for a conventional catalytic cracking unit and about 8% for a unit which is specially adapted for cracking heavy feeds. The use of feeds in which the metallic impurity content is on the upper limit or higher than those mentioned above causes the catalyst to be considerably deactivated, requiring substantial addition of fresh catalyst, and is thus prohibitive for the process and can even render it unworkable. Further, such a process implies the use of substantial quantities of solvent for deasphalting since all the hydrotreated and preferably visbroken product is deasphalted. The use of a single hydrotreatment catalyst limits the performances as regards elimination of metallic impurities to values of less than 75% (Table I, Example II) and/or those of desulphurization to values of no more than 85% (Table I Example II). That technique cannot produce a feed which can be treated using conventional FCC unless the hydrotreated oil, which may have been visbroken, is deasphalted with a C3 type solvent, thus severely limiting the yield.